1. Field of the invention
The present invention relates to a thermal printhead and also a method of making the same.
2. Description of the Related Art
As is well known, a thermal printhead is provided with an elongated heating resistor divided into a multiplicity of heating dots. In operation, the heating dots are selectively energized, so that heat is selectively supplied to transfer ink ribbon or thermosensitive paper for printing required images on recording medium.
Referring to FIG. 11 of the accompanying drawings which shows a related art (not prior art), a thermal printhead may include a heating resistor unit 31A, a signal relay unit 34A and a heat sink 38 supporting these two units. The heating resistor unit 31A includes a primary substrate 31 which is provided with a heating resistor 32 extending longitudinally of the substrate 31. A plurality of drive ICs 33 are mounted on the substrate 31 for controlling the operation of the heating resistor 32 based on externally supplied print data. Though not illustrated, a wiring pattern is formed on the substrate 31 for connecting the drive ICs 33 to the heating resistor 32.
The signal relay unit 34A includes an auxiliary substrate 34 formed with a predetermined wiring pattern (not shown). This non-illustrated wiring pattern is connected to terminal pads 36 formed on the obverse surface of the substrate 34. The terminal pads 36 are spaced from each other in the longitudinal direction of the substrate 34. A connector 37 is attached to the reverse surface of the substrate 34 for making electrical connection between the non-illustrated wiring pattern and an external device or circuit (not shown).
The illustrated thermal printhead also includes a plurality of conductive clip pins 35 for electrically connecting the heating resistor unit 31A to the signal relay unit 34A. Each pin 35 is formed with a generally U-shaped portion and a straight lead portion integral with the U-shaped portion. The U-shaped portion is clipped onto a connection terminal (not shown) formed on the primary substrate 31, while the lead portion is soldered to the relevant one of the terminal pads 36 corresponding to the non-illustrated connection terminal on the substrate 31.
The thermal printhead having the above arrangement is fabricated in the following manner. First, the heating resistor unit 31A and the signal relay unit 34A are prepared. At this stage, the clip pins 35 are attached to the primary substrate 31 of the heating resistor unit 31A but not connected to the signal relay unit 34A yet.
Then, as shown in FIG. 12, the signal relay unit 34A is mounted onto a first chuck member 41. Thereafter, the heating resistor unit 31A is mounted onto a second chuck member 42. The first chuck member 41 may be stationary, while the second chuck member 42 may be movable in sliding engagement with the first chuck member 41 in the directions shown by the double head arrow A in the figure.
For positioning the heating resistor unit 31A to the second chuck member 42, a plurality of upright pins 43 are provided on the second chuck member 42. In use, the substrate 31 of the unit 31A is brought into engagement with the respective pins 33, as shown in FIG. 12. The first chuck member 41, on the other hand, is provided with a positioning piece 46 having an L-shaped cross section, as shown in FIG. 13. (The positioning piece 46 is omitted in FIG. 12 for convenience of illustration.) The positioning piece 46 includes an upright contact surface 46a, which is brought into engagement with a contact edge 34a of the auxiliary substrate 34. With such an arrangement, the signal relay unit 34A is positioned relative to the first chuck member 41.
After the signal relay unit 34A and the heating resistor unit 31A are positioned on the first chuck member 41 and the second chuck member 42, respectively, the second chuck member 42 is moved relative to the first chuck member 41 as shown by the above-mentioned arrow A. In this manner, the lead portion of each clip pin 35 can be aligned with a relevant one of the terminal pads 36 of the signal relay unit 34A. Then, the lead portion of the clip pin 35 is soldered to the terminal pad 36.
Finally, the two units 31A, 34A are removed from the chuck members to be mounted on the heat sink 38 (FIG. 11). At this stage, the connector 37 is attached to the auxiliary substrate 34.
The use of the positioning piece 46 (FIG. 13) may suffer from the following drawback.
Specifically, for improving the production efficiency, the substrate 34 and many other identical substrates may be collectively obtained by cutting a large mother board into small pieces. In this case, the contact edge 34a of the resulting substrate 34 (and the other three edges as well) may often be formed with burrs due to the cutting operation. As readily understood, such burrs will prevent the substrate 34 from coming into proper contact with the contact surface 46a of the positioning piece 46. This means that the substrate 34 will fail to be positioned accurately relative to the first chuck member 41, which may result in inaccurate positioning of the clip pins 35 to the terminal pads 36 on the substrate 34.
Instead of using the illustrated positioning piece 46, at least two positioning holes 50 may be formed in the auxiliary substrate 34, as shown in FIG. 14, by drilling for example. In this case, the first chuck member 41 may be provided with upright protrusions to be fitted into the positioning holes 50. With such an arrangement, the auxiliary substrate 34 can be properly positioned on the first chuck member 41 since fewer burrs are formed on the surface of the drilled positioning holes 50 as compared to the contact edge 34a.
However, as shown in FIG. 14, the positioning holes 50 are formed in the wiring pattern-forming region S of the substrate 34. With such an arrangement, the room for providing the wiring pattern is disadvantageously restricted since the wiring pattern should avoid the location of the positioning holes 50.